This invention relates to a method of manufacturing a transfer mask having, on a transparent substrate, a thin film formed with a transfer pattern and further relates to a method of manufacturing a semiconductor device using this transfer mask.
A transfer mask (photomask) is used for forming a substrate pattern on a semiconductor substrate (wafer) or the like in the manufacture of a semiconductor element or a liquid crystal display. The substrate pattern is formed by photolithography, i.e. by transmitting exposure light from a light source through the transfer mask to transfer a mask pattern of the transfer mask to an object (e.g. a resist film on the semiconductor wafer), then developing the object, and then carrying out etching using the developed object. The transfer mask is obtained, for example, by forming the mask pattern in a mask blank having a chromium light-shielding thin film or a light-semitransmitting (halftone) thin film on a transparent substrate such as a quartz glass substrate.
In general, a transfer mask is subjected to the occurrence of a defect in the manufacture thereof. The defect is, for example, in a pattern, an abnormal projection, abnormal lack, pinhole, scratch, semitransparent defect, size defect, position offset, lost pattern, abnormal gap between adjacent patterns, bridge, abnormal dot, or adhesion of foreign matter. Among them, the extra portion such as the abnormal projection, the abnormal dot, or the bridge is called a black defect and is removed, for example, using a laser, while the lacking portion such as the abnormal lack or the lost pattern is called a white defect and is corrected, for example, by forming a deposited film with high light-shielding property in the defect portion using laser CVD (chemical vapor deposition). When the white defect is corrected in a halftone portion of a halftone mask, the transmittance of a deposited film is controlled to a predetermined halftone value.
The defect is inspected using a transfer mask inspection apparatus. For example, the inspection apparatus emits light from a light source disposed on one side of a transfer mask, detects the intensities of transmitted light beams through a pattern of the transfer mask by a detection section such as an image sensor disposed on the other side of the transfer mask, and converts them into transmitted light quantity distribution data (collective data of position coordinates and transmitted light quantity values).
In general, in order to inspect whether or not there is a defect in a mask pattern of a transfer mask, a die-to-die comparison inspection method or a die-to-database comparison inspection method is used.
The die-to-die comparison inspection method compares the detection results of the transmitted light quantities through two real patterns of the same shape which are disposed at different positions of a transfer mask. For example, using the above-mentioned inspection apparatus, transmitted light beams through the two real patterns of the same shape disposed at the different positions of the transfer mask are detected and converted into data of transmitted light quantity distributions for the two real patterns. Then, these transmitted light quantity distribution data of the two real patterns are compared with each other by a control circuit or the like to record coordinates of inconsistent portions and differences not less than a threshold value, thereby detecting the position and size of a defect in the mask pattern.
On the other hand, the die-to-database comparison inspection method makes a comparison between a design pattern stored in a database and the detection result of the transmitted light quantities through a real pattern formed on a transfer mask. For example, from data of the design pattern stored in the database, transmitted light quantity distribution data of a comparative mask pattern is produced by simulation assuming that a real pattern with no defect is formed on a transfer mask. Then, the transmitted light quantity distribution data of the comparative mask pattern and transmitted light quantity distribution data obtained by detecting transmitted light beams through the real pattern of the transfer mask are compared with each other by a control unit or the like to record coordinates of inconsistent portions and differences not less than a threshold value, thereby detecting the position and size of a defect in the mask pattern.
In the case of a mask pattern in which a halftone pattern portion and a light-shielding pattern portion are provided on a transparent substrate (glass substrate) and a region other than those pattern portions serves as a light-transmitting pattern portion where the transparent substrate is exposed almost as it is, there is a case where white and black defects are present in the mask pattern. In this case, with respect to the white or black defect in the light-shielding pattern portion, when the mask pattern is inspected by the above-mentioned inspection method, since the amplitude from a difference data reference value at a defect data position is large (the magnitude of decrease or increase in transmitted light quantity from the difference data reference value is large), it is possible to set a threshold value away from the difference data reference value and thus detection is easy.
On the other hand, with respect to the white or black defect in the halftone pattern portion, since the amplitude from a difference data reference value at a defect data position is small, it is necessary to set a threshold value close to the difference data reference value. However, since data has noise, if the threshold value is set close to the difference data reference value, when the level of the noise is high, a portion, which is not a defect, may be erroneously detected as a defect. For this reason, the threshold value cannot be set close to the difference data reference value. In order to accurately detect a defect even in such a halftone pattern portion, it is proposed to amplify difference data of the halftone pattern portion to a level equivalent to that of difference data of the light-shielding pattern portion (see, e.g. JP-A-2007-240517 (Patent Document 1)).
It is also known that an internal defect (optically nonuniform portion) that affects exposure light in the use of a transfer mask is possibly present in a transparent substrate (glass substrate) which is used as a substrate of the transfer mask. In recent years, the wavelength of exposure light for use in photolithography has been shortened so that ArF excimer laser light has been used more frequently. Under these circumstances, it has been found that there is a case where an internal defect of the type that does not cause a local reduction in transmittance with respect to light having a wavelength longer than 200 nm, such as KrF excimer laser light, but causes a local reduction in transmittance with respect to light having a wavelength of 200 nm or less, such as ArF excimer laser light, is present in a transparent substrate. An internal defect inspection for detecting such an internal defect present in the transparent substrate is often carried out before forming a thin film on the transparent substrate. In JP-A-2007-86050 (Patent Document 2), a transparent substrate is inspected for the presence or absence of an internal defect by irradiating inspection light having a wavelength of 200 nm or less to the transparent substrate and detecting light having a wavelength longer than that of the inspection light from the transparent substrate.